The Baltimore Orioles are as puzzled as anyone else. Ben McDonald has become a mystery, and the club's once-lustrous pitching future won't be bright again until someone figures him out.

It is a one-minute mystery -- one minute McDonald pitches well, the next minute he's got everyone wondering what all the excitement was about in the first place.

Last night fell into the second category. McDonald gave up 10 hits over 7 1/3 innings and dropped a 4-2 decision to the struggling Milwaukee Brewers at Memorial Stadium.

Brewers right-hander Jaime Navarro (9-9) ran his career record against the Orioles to 5-0 with a complete-game performance in which he gave up two runs on nine hits. McDonald gave up all four Brewers runs and lost for the third time in his past four decisions to fall to 5-6, which is not exactly what the Orioles had in mind for his first full season in the major leagues.

The Orioles still are waiting to see McDonald pitch the way he did in the second half of the 1990 season, but he has yet to display the consistency that marked his arrival in the major-league starting rotation.

"I think it's mechanical," manager John Oates said. "We've had him back off on his fastball, figuring that throwing it at 88 or 89 mph with good control is better than throwing it harder without. We've talked about different grips on the curveball. We've worked on a lot of things, and we're going to try all the possibilities until he finds what clicks. Until we push the right button, we'll keep working."

It wasn't a terrible game. It just wasn't what the baseball world has come to expect from one of the most highly touted college prospects in baseball history. McDonald said he is being held to a higher standard than the other pitchers on the Orioles staff, and he may be right.

"Sure, I think so," he said, "but there is nothing you can do about that. I was the No. 1 pick, and there are expectations that come with that. That's something I've had to deal with, and that's the way it's going to be for a lot of other young players."

He was coming off a 5 2/3 -inning appearance in which he gave up no runs and just four hits, but his performance was not quite as clean as the pitching line might indicate. He struggled with his control and walked four before allowing four relievers to complete a 3-0 shutout of the Chicago White Sox.

Still, that was his second decent outing in a row, which had to be somewhat encouraging considering the 16 earned runs he had given up in his previous 23 innings (6.26 ERA).

This time, he gave up a leadoff triple to Paul Molitor in the first inning and walked No. 2 hitter Willie Randolph on four pitches before getting out of the jam with a shallow fly ball and a double-play grounder. He was not so fortunate in the second, when he walked the leadoff hitter and allowed an RBI single to B. J. Surhoff.

McDonald is becoming more and more a victim of the great expectations he created with his 5-0 start in 1990, and he has failed to live up to them on all but a couple of occasions this year.

"He is not pitching the way we've seen him pitch," Oates said. "The way he pitched against Detroit [eight innings, two hits on July 1] and Chicago [a four-hitter in his starting debut last year], when you see him pitch that kind of ballgame, that's how you judge him because he can do that a lot more often than once or twice a season.

"Ben McDonald is capable of overmatching hitters."

That has not been apparent since he struck out eight in an eight-inning performance against the Oakland Athletics on July 12.

Where McDonald once was overpowering, he has become all too hittable, giving up 42 hits and nine walks in his past 31 innings (five starts). He worked with runners on base in four of the first five innings last night and fell behind for good when the Brewers reeled off five straight singles to score three runs in the fifth.

He can see what's going wrong, but he has yet to correct it.

"Last year, I was getting the breaking ball over the plate," he said, "and I was able to throw it behind on the count. This year, guys know you're struggling, so they can wait on the fastball.

"This has been a tough year, being hurt and trying to come back twice. I got into some bad mechanics throwing when my arm was bothering me."

The Orioles had tied the game in the bottom of the third on a leadoff double by Juan Bell and a two-out RBI double Cal Ripken, but the game slipped away in the Brewers' five-hit fifth.

Bell led off the Orioles' half of the fifth with another double and scored on a one-out single by Joe Orsulak, but that was all that Navarro would allow in his seventh complete game of the year.

It was the third straight multiple-hit game for Bell, who has been one of the club's hottest hitters the past two weeks. He was batting .130 on July 24, but has gone 15-for-41 to raise his average to .200.